Most reported releases of hazardous materials occur on the highways. Statistical analysis reveals an unexpected increase, starting around May 2001, in the underlying trend in the number of highway hazardous materials incidents. This increase can not be explained by variability in the seasonal behavior. It may be due to DOT outreach efforts to improve reporting. BTS will be monitoring the incident data over the next few months to determine if the increase is temporary or not.

NOTE: The current value is compared to the value from the same period in the previous year to account for seasonality.

Incident reporting requirements were extended to intrastate motor carriers on October 1, 1998, which may partly explain the subsequent increased volume of reports. Beginning in April 1993, there was sharp improvement in reporting of incidents by small package carriers.

A reported incident is a report of any unintentional release of hazardous material while in transportation (including loading, unloading, and temporary storage). It excludes pipeline and bulk shipments by water, which are reported separately.

A trendline has been provided for highway incidents. The trend has been calculated through a statistical procedure called Structural Modeling, in which the time series under study is decomposed into seasonal, trend and irregular components. For further information on this statistical procedure, see: S.J. Koopman, et al., Structural Time Series Analyser, Modeller and Predictor (STAMP), London: Timberlake Consultants Ltd., 2000

SOURCE: U. S. Department of Transportation, Research and Special Program Administration, Office of Hazardous Materials, Planning and Analysis, Hazardous Materials Information System data obtained through personal communication.